Some one please help..i got a 2000 focus with the check engine light on all the time. I took it to autozone and they checked it and said it was something to do with emission control. So I replaced both o2 sensors, new gas cap, and i had the check engine light reset. About 300 miles it comes back on..Any suggestions

Do you remember the DTC # from the scan? If it was an EGR code, then it may be covered under warranty if it is the DPFE sensor.

That sounds like a great idea. you can get all of the codes online so you know what your looking at and how to do it right. It is not hard to do. Mine did that for a bit, it was a sensor. I hate computers in cars. There is another forum that has that info you need, just yahoo it. That will save you time and money, since you know what to tell the mechanic to replace.